1. Field of the Invention
The present invention relates to a coating method, and more particularly relates to a magnetic dispersion coating method having high shear regions. Specifically, the present invention is directed to a coating apparatus wherein regions of high shear are provided both before and after a coating application point for applying magnetic dispersions on a continuously traveling flexible support material.
2. Description of Background Art
With the advent of computerized data processing equipment and digitized information, there also came a need and requirement for reliable storage media to store data in analog or digital form. Currently, there are a variety of different storage media available for storage of digitized information, e.g., magnetic tapes, magnetic flexible disks, magnetic hard disks, and optical storage media such as the so-called compact discs, etc.
Magnetic tapes and magnetic flexible disks enjoy widespread use due to their economy and availability. These media are manufactured by coating magnetic dispersions directly onto moving flexible supports. In the case of magnetic tapes, application of a coating of liquid magnetic dispersions is generally followed by an orientation of the magnetic particles, whereas in the case of magnetic disks, application of a coating of liquid magnetic dispersions is generally followed by disorientation of magnetic particles. The coating is then dried by convection or by electron beam curing and then calendered. Optional finishing steps include curing, burnishing, cleaning and/or sciving, depending on the parameters required or the particular intended use for the recording media.
Concurrent with the development of more powerful, computerized data processing equipment that handle both analog and digital data, there is a movement towards smaller and faster data storage devices having increased data storage capacities and increased data storage densities. For example, recently introduced digital data tape backup units are capable of storing enormous amounts of digitized data, e.g., 2 gigabytes, on magnetic tapes using, for example, 8 mm videocassettes. The most common way to effect the storage of such large amounts of data onto such small storage media is to increase the densities at which the data is recorded on the recording media.
The reliable storage and retrieval of information to and from data storage devices at any data transfer rate is critical in any data processing system. Thus, when high speed data transfer rate is combined with high density storage of data, the quality and integrity of the data storage media become all the more critical. Accordingly, the data storage media must meet certain levels of quality and performance parameters in order to satisfy and meet the requirements for safe, reliable and effective storage of data at high densities and at high data transfer rates while also allowing for effective retrieval of data under those same parameters.
With the movement towards smaller and faster data storage devices having increased data storage capacities and increased data storage densities, the physical size of each stored bit, the principal element of digitally stored information, necessarily becomes smaller as well. For magnetic recording media, the track-width of recorded magnetization becomes narrower and the wavelength of the recorded magnetization pattern becomes shorter. Thus, the uniformity of the magnetic properties of the magnetic coating must be at a level and at a scale sufficient to effectively and reliably record the magnetization pattern at the narrower track-widths and shorter wavelengths, i.e., the uniformity of the magnetic properties has to be measured at the same scale as the recorded magnetization. The surface quality, i.e., smoothness, of the magnetic recording media is also very critical to high performance media because the recording depth of the signals recorded onto the magnetic media is proportional to its wavelength. However, when the magnetic properties of magnetic recording media are measured on BH-loop tracers or vibrating sample magnetometers (VSM), the measurements reveal or characterize only bulk magnetic properties of the magnetic recording media as a whole and generally do not allow a determination or prediction of the reliability and performance of the recording media during actual use.
In any data storage/retrieval device that uses magnetic storage media, the signal-to-noise ratio is a critical performance parameter. Based on experimental and theoretical studies, it is known that the quality and operational characteristics of magnetic recording media, e.g., electronic noise of signal read-back from the storage media, are affected not only by the physical characteristics of the magnetic coating in terms of, for example, uniformity of thickness and uniformity of surface texture, but also by the magnetic character of the coating in terms of the uniformity of the magnetic properties of the coating, i.e., uniformity and quality of the magnetic dispersion. In general, the performance of the data storage/retrieval device is limited by the signal-to-noise ratio of the magnetic media itself. An increase in signal strength and/or a decrease in noise improves the signal-to-noise ratio of the magnetic media.
With the current trend in computer and data processing technology (including video and audio equipment) towards increased recording densities as well as increased power and complexity with the capabilities of processing increasing amounts of data over time coupled with the trend towards smaller size, there exists a need for storage media that satisfies the performance requirements for such equipment in terms of supporting the storage of data at higher densities as well as the transfer of data at higher data transfer rates during writing and reading operations.
A conventional coating die used in making magnetic recording media 10 is shown in FIG. 1 and generally comprises at least two die members 12, 14 opposedly and appropriately fastened against each other to define an extrusion passageway 16 through which magnetic dispersion is extruded out of extrusion opening 18 onto the surface of a moving flexible support 40. The dies are typically made from non-magnetic stainless steel although other materials such as non-magnetic metal alloys, ceramics and ceramic composites may be used. Although magnetic steels may be used in making the dies, such dies must be routinely demagnetized. At least one feed channel 19 is provided in at least one of the die members and extends through the body of the die to communicate with a magnetic dispersion source (not shown) and a distribution chamber 21. The extrusion passageway 16 extends from the distribution chamber 21 to an extrusion opening 18 through which the dispersion is applied on a moving support web 40. A severe drawback associated with this type of conventional coating device is the difficulty in controlling the shear rate of the magnetic dispersion traveling from the feed channel 19 to the distribution chamber 21 and through the extrusion passageway 16 before application on a moving support web 40. As a result, there is always a tendency for the magnetic dispersion to agglomerate as it passes from the feed channel 19 to the distribution chamber 21 and through the extrusion passageway 16 before exiting the extrusion outlet 18. Moreover, the magnetic dispersions that exit the extrusion outlet 18 may not be applied evenly and uniformly across the expanse of the coating layer. The net effect is that uniform coatings of magnetic dispersions in terms of both surface smoothness and magnetic properties are very difficult to attain.
As illustrated in FIG. 10, it is known to use a rotating bar 24 inside a cylindrical cavity 23 of a coating apparatus 10 to maintain high shear rates on magnetic dispersions. For example, U.S. Pat. No. 3,227,136 addresses the problems of uncontrollable shear rate and agglomeration by providing a cylindrical rotor, i.e., a rotating bar, in the cavity of an extrusion coating device to shear the magnetic dispersion prior to application on the moving web of support in order to maintain a relatively constant viscosity across the web. This prior art patent recognizes the importance of maintaining high shear rate conditions on magnetic dispersions as necessary to prevent agglomeration of the dispersed particles.
U.S. Pat. No. 3,479,989 identifies certain shortcomings of U.S. Pat. No. 3,227,136 as, inter alia, the presence of an "entrance defect", which appears under nearly all operating conditions as a line in the coated product corresponding in position to the entrance port through which the dispersion enters the chamber in the die body. Other identified shortcomings include limited flow capacity of the die and the high pressure drop in the dispersion in passing from the die entrance to the remote portions of the chamber. U.S. Pat. No. 3,479,989 addresses these shortcomings by providing a hollow, cylindrical distribution chamber formed within the body of a die, a metering slot terminating in an extrusion orifice and, extending along the length of the chamber and arcuately from the chamber, at least one slotted, or otherwise perforated, tubular rotor mounted within the chamber and having an outer diameter slightly less than the diameter of the chamber, and an inlet for directing a dispersion into an end of the rotor. A stationary shaft is mounted concentrically inside of the rotor to define an annular shear chamber with the inner surface of the rotor.
U.S. Pat. No. 4,828,779, also utilizes a rotating bar to maintain a uniform viscosity of the dispersion across the slot opening as it is applied on the moving flexible support web. In this patent, the coating solution or dispersion flows into an extruder through a solution- or dispersion-receiving inlet after which the coating solution or dispersion is sheared by rotor or rotating bar in an L-shaped region formed by the rotor and a pocket where there is a variance in viscosity, so that the viscosity of the coating solution is made substantially equal to the final viscosity of the coating solution in the pocket just before the coating solution reaches the end of the slit.
While the above-described prior art methods and apparatus are sufficient for producing magnetic storage media that satisfy the requirements and criteria for a relatively low density storage media, they are not capable of providing a magnetic dispersion coating of sufficient uniformity in terms of uniformity of magnetic characteristics and smoothness of surface finish to satisfy the requirements and criteria for a relatively high density storage media. Furthermore, the above-described prior art methods do not provide any means of shearing the dispersion after the application point. For example, the signal-to-noise ratio of magnetic storage media made using these prior art methods and apparatus is well below the level required for applications involving high density storage.
Doctoring blades have been used in coating apparatus for the purpose of metering the amount of coating solution being applied to a moving support before the coating solution is applied onto the support. For example, U.S. Pat. No. 4,424,762 describes a doctor edge made of cemented carbide. The doctor edge has a doctor surface bent towards the support so that the doctor edge is triangular in section, and the doctor edge is set close to the support in such a manner that the support is bent substantially triangular, therefore metering the amount of coating solution applied.
U.S. Pat. No. 3,584,600 discloses a multiple doctor blade coating apparatus for preparing photographic layers of photographic coating compositions wherein the final doctor blade has a surface dimension that is at least five times that of the preceding doctor blade and is used to meter the final, top coating composition. By use of this multiple doctor blade coating apparatus, it is possible to reduce the total thickness of the multiple layer coating by about 30 percent at spacings comparable to prior art.
U.S. Pat. No. 3,526,528 also discloses a coating apparatus having multiple doctoring blades for the purpose of coating a plurality of photographic emulsion layers on a traveling support, wherein each doctor blade is placed equidistant from the surface of the traveling web.
In U.S. Pat. No. 3,063,868, an air doctor knife is used to meter the amount of coating composition being applied to continuous webs. The leading edge of a shoe can also be used to mechanically doctor the layer with or without the smoothing action of the air doctor knife.
In all of the above-described prior art, the doctoring blade is used to meter the coating composition onto the support. The doctoring blades do not provide increased shear and are not intended to do so, since any increased shear would destroy the desired integrity of individual photographic layers. The use of doctor blades or edges also generates bead lines or streaks on the surface of the magnetic coating layer. Additionally, the buildup of excess magnetic coating composition in front of the doctoring blade requires removal in order to avoid solid or sticky buildup on the doctoring blade. Moreover, the creation of such excess magnetic coating composition is, in the first instance, a waste of the coating composition and, secondly, a cleanup problem. In the case of an air doctor knife, it is not possible to adapt such a knife for use on magnetic coating compositions because an air doctor knife would result in a flash drying of the coating composition, and such a result would create a magnetic film coating whose surface reflects the imperfections at the time of the flash drying while also generating an excess of coating material that must be collected and disposed of properly.
Accordingly, there has been a need for an apparatus for producing magnetic recording media that satisfies and meets the aforementioned needs, requirements and criteria without the shortcomings and disadvantages of the prior art in a safe and cost-effective manner. More specifically, the present invention is directed to a coating apparatus having a concave lip extending arcuately for applying magnetic dispersions on a continuously traveling flexible support material. The concave lip of the present invention is not a doctoring blade and is distinguished thereover because the concave lip is not used to meter the coating composition and, instead, is used to shear the coating composition as it is applied onto the support material.
A need has therefore existed for a coating apparatus that evenly and uniformly coats a coating media onto a continuously traveling flexible material. There has also been a need to provide a coating apparatus to apply an even and uniform magnetic dispersion coating on a flexible substrate to produce particulate magnetic media with superior dispersion qualities. There has still been another need to provide a coating apparatus that permits the even and uniform application of uniform magnetic dispersions on a flexible substrate under high shear conditions.
There has also been a need to provide a coating apparatus to apply onto a moving flexible substrate a magnetic dispersion coating layer having uniform magnetic characteristics over the entirety of the coating layer and having a uniform, smooth surface finish. Still another need has been to provide a coating apparatus to apply a magnetic coating composition onto a moving substrate web without generating any excess waste coating composition during the application of the coating composition.
There has also been a need to provide a new and useful method of applying a magnetic dispersion coating onto a moving substrate web. Moreover, there has been a need to provide a new and useful method of making magnetic recording media.